This invention relates in general to gas turbine combustion systems and specifically to a method of operating a gas turbine combustion system at significantly lower load conditions while having stable combustion and lower emissions.
Gas turbine engines typically include a compressor, one or more combustors each having a fuel injection system, and a turbine section. In an engine having a plurality of combustors, they are typically arranged in an annular array about the engine and most typically interconnected for the purposes of ignition. The compressor raises the pressure of inlet air, and then directs it to the combustors, where it is used to cool the combustion chamber walls as well to provide air for the combustion process. In the combustion chamber, compressed air is mixed with a fuel and the mixture is ignited by an ignition source to produce hot combustion gases. Typically, ignition occurs within a single chamber, and for engines with multiple combustors, the flame passes through tubes interconnecting the combustors to ignite the fuel air mixture in the adjacent combustor. This process continues around the engine until fuel-air mixtures in all combustors have been ignited. The hot gases resulting from the combustion process are then directed to drive a turbine. For land-based gas turbines, whose primary purpose is to generate electricity, a generator is coupled to the turbine shaft such that the turbine drives the generator.
While a full load condition is the most common operating point for land-based gas turbines used for generating electricity, often times electricity demands do not require the full load of the generator, and the operator desires to operate the engine at a lower load setting, such that only the load demanded is produced, thereby saving fuel costs. Combustion systems of the prior art have been known to become unstable at lower load settings while also producing unacceptable levels of carbon monoxide (CO) and oxides of nitrogen (NOx) at these lower load settings, especially below 50% load. This is primarily due to the fact that most combustion systems are staged for most efficient operation at high load settings and therefore operate less efficiently at lower load settings. Furthermore, it is well known in the art of combustion that lower emissions are achieved through premixing air and fuel together prior to combustion, instead of through diffusion, and therefore premixing is the preferred method of combustion for highest efficiency and lowest emissions. However, advancements have been made with regards to fuel staging in an effort to lower emissions. For example, U.S. Pat. No. 5,551,228 discloses a method of operating a combustor involving assymetrical fuel staging within a combustor and axially staging fuel injection within a single fuel nozzle for reducing emissions. Furthermore, U.S. Pat. No. 5,924,275 discloses a method of operating a combustor that utilizes the addition of a center pilot nozzle in combination with the previously mentioned assymetrical fuel staging to provide reduced emissions at lower load conditions. While this staging method and combustor configuration is an enhancement, it is still limited in turndown capability, such that in order to achieve turndown to low, part-load settings, the combustor must often revert to the higher emissions diffusion mode and not operate in the lower emissions premix mode. An effort to overcome the shortcomings of the prior art was disclosed by co-pending U.S. patent application Ser. No. 10/437,748 assigned to the same assignee as the present invention. However, this prior patent application for staging fuel to produce low emissions at low load settings was directed to a combustor configuration having a can-annular configuration in which adjacent combustors communicated with each other via crossfire tubes.
The combination of potentially unstable combustion and higher emissions often times prevents engine operators from running engines at lower load settings, forcing the engines to either run at higher settings, thereby burning additional fuel, or shutting down, and thereby losing valuable revenue that could be generated from the part-load demand. A further problem with shutting down the engine is the additional cycles that are incurred by the engine hardware. A cycle is commonly defined as the engine passing through the normal operating envelope and thereby exposing the engine hardware to a complete cycle of pressures and temperatures that over time cause wear to the engine hardware. Engine manufacturers typically rate hardware life in terms of operating hours or equivalent operating cycles. Therefore, incurring additional cycles can reduce hardware life requiring premature repair or replacement at the expense of the engine operator.
What is needed is a system that can provide flame stability and low emissions benefits throughout the full operating conditions of the gas turbine engine for a combustion system in a can orientation, including a low part-load condition. This system should be one that can be efficiently operated at lower load conditions, thereby eliminating the wasted fuel when high load operation is not demanded or incurring the additional cycles on the engine hardware when shutting down.